ABSTRACT This Project's goal is to confirm reported mechanisms of injury caused by methyl isocyanate (MIC) exposure using a new airway delivery model and to test the efficacy of potential rescue countermeasures in mitigating this injury. To date, there are no effective treatments for MIC toxicity. MIC causes profound upper airway and lung injuries. MIC is a colorless, foul smelling, dense, highly reactive irritant chemical used to produce carbamate pesticides and herbicides, plastics, rubbers, paints and adhesives. Millions of tons of isocyanates are produced annually for polyurethane production In December 1984, MIC was accidentally released over Bhopal, India, in arguably the worst inhalation disaster in history. The accident killed >3,000 persons overnight, more in ensuing weeks, and left up to 570,000 surviving victims. There have been two `near miss' MIC releases in the US in the past decade, and industrial MIC still exists in the US. In Tianjin, China, there was a recent large spill of the related compound toluene diisocyanate (TDI), along with other toxic chemicals, which left >112 dead, and injured >700. Based on known pathologic, chemical and physiologic responses to MIC, we hypothesized that MIC will cause progressive damage to upper and lower airway epithelium, followed by fibrin deposition. Because of MIC's great reactivity, we hypothesize also that rescue agents that target relevant TRP (Transient Receptor Potential ion potential irritant receptor) pathways, that clear airway fibrin, even when the latter is given in delayed fashion, or provide thiol substrates or restore GSH will be effective rescue countermeasures and improve airway obstruction, gas exchange, and mitigate lung injury. To evaluate these possibilities, we have constructed a system for inhalation exposure of rats to MIC vapor. This Project will investigate three classes of potential countermeasures, TRP channel antagonists, plasminogen activators, and thiol compounds, that should be effective individually and in combination to diminish MIC-induced airway injuries. These agents can be used both to further investigate mechanism(s) of acute airway injury due to MIC inhalation, as well as for therapeutic development. Our preliminary data indicate that certain TRP inhibitors, as well as tissue plasminogen activator (tPA), can decrease mortality in rats exposed acutely to MIC, very likely by different mechanisms. In addition, others have found NAC and GSH effective in other in vivo MIC toxicity models. We suggest that the timing of administration will impact efficacy of the different classes of therapies. Hence, thiol compounds likely would require early administration due to extreme reactivity of MIC, while TRP channel antagonists could potentially be given within 2 h of initial exposure, and tPA might be delayed up to 4-6 h. Based on possible real life scenarios, such interventions likely could be useful for rescue treatment of industrial workers, persons residing in the area of storage facilities, and first responders. For the most promising rescue agent(s) these studies should enable a pre-IND meeting with FDA.